The present invention relates to a magnetic circuit for a loudspeaker wherein the bottom surface of a form provided with a rectangular parallelepiped cavity penetrating two opposed lateral faces of a substantially rectangular parallelepiped is connected with the bottom surface of a sintered magnet of substantially rectangular parallelepiped configuration smaller than said rectangular yoke, and a polepiece connected to the top surface of said rectangular sintered magnet is opposed to the annular edge surface of a small through hole formed in the top surface of said rectangular yoke so as to form a magnetic air gap in which a movable coil can be positioned.
A conventional magnetic circuit for speaker employing a rectangular yoke has generally utilized an alnico cylindrical conventional magnet. In these conventional magnetic circuits, the bottom internal surface of the rectangular yoke having a rectangular parallelepiped cavity is connected with the bottom surface of a cylindrical alnico magnet, the top surface of the magnet is connected with a polepiece that extends through a small through-hole in the upper yoke wall. An annular magnetic gap region is formed between said polepiece and the inside edge of the through hole in the top surface of the rectangular yoke is provided with a mobile coil (not shown). Comparing the recoil permeability, .mu..sub.r of alnico magnet is about 3.0, that of ferrite .mu..sub.r .apprxeq.1.06 and that of rare earths=1.01; and thus the recoil permeability of an alnico magnet is about three times as large as a ferrite or rare earth magnet.
An alnico magnet suffers magnetic leakage of about 20% from the lateral wall. In order to enclose the leaked flux generated by the alnico magnet in the internal hollow formed by the rectangular yoke, the outer diameter of the alnico magnet should have been about 50-70% of the width or length of the rectangular yoke. When an alnico magnet is used as a source of magnetomotive force for this purpose, the cross sectinal area had to be much smaller than the bottom internal area of the rectangular yoke. On the other hand, when a sintered magnet such as ferrite magnet and rare earth-cobalt magnet having a recoil permeability larger than 1.0 and less than 1.2 is used, even a magnet of the dimension equal to that of the internal hollow region of the rectangular yoke can be utilized and which prevents substantially the leakage of magnetic flux past the flank of the rectangular yoke and permits the maximum utilization of the internal space of the rectangular yoke.
The present invention was realized on the ground of the above point of view. When the ferrite magnet is used as the source of magnetomotive force, a large merit is found as follows: recently the source of cobalt, one of the principal materials for alnico magnet, has been almost exhausted and its price has risen. On the contrary, the ferrite magnet which is abundant on the earth and easily obtainable can be utilized as a substitute for alnico magnet. This is desirable from the view point of saving natural resources. In spite of this situation the inventors are unaware of a successful example of the manufacture of a magnetic circuit for a speaker exhibiting a performance equal to or larger than that of magnetic circuit of conventional cylindrical alnico magnet and the rectangular yoke design by utilizing the rectangular yoke and the ferrite magnet. That is why one of the representative magnetic oxide material i.e. the ferrite magnet has magnetic characteristics such as maximum energy product lower than those of alnico magnet. As a result, a magnet much larger than alnico magnet must be used in order to obtain an air gap magnetic flux of Bg of the value equal to that of an alnico magnet of the same shape. Such large magnetic oxide magnet may not be able to be inserted in the space reserved for the magnetic circuit for the speaker.
In general, when a ferrite magnet is used in a magnetic circuit, it is used in a speaker magnetic circuit of the external magnet type. However, in radio and TV, the external magnet leakage of magnetic circuit for speaker is of great significance, and this ferrite magnet of external magnet type cannot be employed.